Propylene allylic intermediate

Ammoxidation of propylene is considered under oxidation reactions because it is thought that a common allylic intermediate is formed in both the oxidation and ammoxidation of propylene to acrolein and to acrylonitrile, respectively. 

Much work has been invested to reveal the mechanism by which propylene is catalytically oxidized to acrolein over the heterogeneous catalyst surface. Isotope labeling experiments by Sachtler and DeBoer revealed the presence of an allylic intermediate in the oxidation of propylene to acrolein over bismuth molybdate. In these experiments, propylene was tagged once at Ci, another time at C2 and the third time at C3. 

A proposed mechanism for the oxidation of propylene to acrolein is by a first step abstraction of an allylic hydrogen from an adsorbed propylene by an oxygen anion from the catalytic lattice to form an allylic intermediate ... 

The nickel catalyst under the condition for the 1 1 codimerization is not known to dimerize or polymerize ethylene, although a similar catalyst system has been known to dimerize propylene (26, 27) via a w-allyl intermediate. 

Temperature Programmed Reaction. Examination of another redox system, propylene oxidation on M0O3, provides further insight. It is well accepted that propylene oxidation on molybdenum-based catalysts proceeds through formation of allylic intermediates. From isotopic studies it has been demonstrated that formation of the allylic intermediate is rate-determining (H/D effect), and that a symmetric allylic species is formed ( C labelling). 

Kinetics of reaction must be considered when attempting to postulate mechanisms, but kinetic equations alone are unreliable in fixing mechanism. For example, in the oxidation of propylene to acrolein, cuprous oxide and bismuth molybdate have very different kinetics, yet the studies of Voge, Wagner, and Stevenson (18), and especially of Adams and Jennings (1, 2) show that in both cases the mechanism is removal of an H atom from the CH3 group to form an allylic intermediate, from which a second H atom is removed before the O atom is added. The orders of the reactions and the apparent optimum catalysts (16) are as follows ... 

Russian workers using 14C-labeled propylene, acrolein, and acetaldehyde (29-31) have determined that carbon oxides are formed chiefly through the further oxidation of acrolein and that acetaldehyde and formaldehyde are produced either from acrolein or directly from the symmetrical 7r-allylic intermediate. These two saturated aldehydes can then undergo further oxidation (about 20 times more rapidly than acrolein) to CO and COj. The overall scheme proposed is given in Fig. 4. 

Keulks et al. (32) have also concluded from the oxidation of 14C-labeled and unlabeled acrolein that carbon dioxide is formed almost exclusively from the further oxidation of acrolein. Thus, it can be seen that the initial step in the formation of carbon dioxide and the other side products of propylene oxidation is the formation of a symmetrical 7r-allyl intermediate. This 7r-allylic intermediate is responsible for both the selective and nonse-lective oxidation of propylene, the course of the overall reaction depending on the subsequent reaction pathway of the allylic species. 

Further evidence supporting the bismuth center as a site of propylene activation comes from the analysis of the rates of formation and product distribution of propylene oxidation over bismuth oxide, bismuth molybdate, and molybdenum oxide. Bismuth molybdate is highly active and selective for the conversion of propylene to acrolein. However, the interaction of propylene with its component oxides yields very different results. Haber and Grzybowska (//. ), Swift et al. 114), and Solymosi and Bozso 115) showed that in the absence of oxygen, propylene is converted to 1,5-hexadiene over bismuth oxide with good selectivity and at a high rate, whereas molybdenum oxide is known to be a fairly selective but a nonactive catalyst for acrolein formation. The formation of 1,5-hexadiene over bismuth oxide can be explained if the adsorption of propylene on a bismuth site yields a ir-allylic species. Two of these allylic intermediates can then combine to give 1,5-hexadiene. 

These observations suggest a reaction scheme for bismuth molybdate catalysts where the allylic species is formed initially at a bismuth center and then reacts further at a molybdenum site to produce acrolein. Thus, once the allylic complex is formed, the MoO polyhedra are highly active and selective for acrolein formation. This hypothesis was tested by investigating the oxidation of bromoallyl (C3HjsBr) over molybdenum oxide 116). Since the C—Br bond in bromoallyl is much weaker than the C—H bond in propylene, the ease of formation of the allylic species should be significantly enhanced with bromoallyl compared with propylene. If the initial propylene activation occurs on bismuth, then the reaction of bromoallyl over molybdenum oxide should approach the activity and selectivity of propylene over bismuth molybdate. This was the observed result, and the authors concluded that the bismuth site was responsible for the formation of the allylic intermediate. 

Clark and Cook 71) disproportionated [1-14C] propylene and [2-14C] propylene over cobalt oxide-molybdate-alumina catalyst. At 60 °C their results were consistent with those reported Mol and coworkers, confirming the four-center mechanism. At temperatures above 60 °C, double-bond isomerization activity of the cobalt-molybdate catalyst became a factor and at 160 °C nearly one-half of [l-l4C] propylene had isomerized to [3-14C] propylene prior to disproportionation. The authors note that at temperatures where isomerization does not occur, the possibility of a jr-allyl intermediate appears to be excluded however, at higher temperatures, the 77-allyl mechanism cannot be so easily dismissed. 

In allylic oxidation, an olefin (usually propylene) is activated by the abstraction of a hydrogen a to the double bond to produce an allylic intermediate in the rate-determining step (Scheme 1). This intermediate can be intercepted by catalyst lattice oxygen to form acrolein or acrylic acid, lattice oxygen in the presence of ammonia to form acrylonitrile, HX to form an allyl-substituted olefin, or it can dimerize to form 1,5-hexadiene. If an olefin containing a jS-hydrogen is used, loss of H from the allylic intermediate occurs faster than O insertion, to form a diene with the same number of carbons. For example, butadiene is fonned from butene. 

The most well accepted feature of the mechanism is the formation of an allylic intermediate via a-hydrogen abstraction from propylene in the ratedetermining step. The structure of this intermediate and its subsequent steps involved in its conversion to selective products are much less well understood. It has been suggested from deuterium-labeling studies (77) that this intermediate undergoes a second hydrogen abstraction followed by O (oxidation) or N insertion (ammoxidation) (Scheme 3). The formation of an... 

N-allyl species and two hydrogen abstractions account for acrylonitrile formation. Thus, although allyl radicals are probably not the selective intermediate in propylene oxidation and ammoxidation, they can form acrolein or acrylonitrile via these selective O- or N-allyl intermediates. 

The Grasselli et al. mechanism (27) is based on a consideration of the reactions of both propylene, which is incorporated in the mechanism of the rate-determining step, and the allylic intermediate, independently generated in situ from allyl radicals or allyl alcohol, which give information concerning the steps after initial a-hydrogen abstraction. 

The role of Te is also very important in these catalysts. Grasselli suggested that an a-hydrogen ahstraction of the surface-adsorbed propylene can easily take place on Te atoms, leading to 7t-allyl intermediates, which react further with NH surface species to form acrylonitrile [3]. This correlates well with the increase in acrylonitrile selectivily with Te loading. 

Confirmation and extension of this mechanism was obtained by Adams and Jennings 97,109) using propylene labeled with deuterium in various positions. Ammonia was also added to the feed to produce acrylonitrile. After reasonable corrections for small effects of propylene isomerization and deuterium exchange, the results were in quantitative agreement with a model in which allylic hydrogen abstraction occurs to form an allylic intermediate followed by hydrogen abstraction from either end. The model is illustrated by the following scheme for l-propene-3d. 

Surface Reaction Mechanism. The mechanism of catalytic alkene ammoxidation is invariably linked to allylic oxidation chemistry. Allylic oxidation is the selective oxidation of an alkene at the allylic carbon position. Selective allylic oxidation and ammoxidation proceed by abstraction of the hydrogen from the carbon positioned a to the carbon-carbon double bond. This produces an allylic intermediate in the rate-determining step. In the case where propylene is the hydrocarbon, the reaction is as follows ... 

Fig. 3. Proposed structure of active site of bismuth molybdate propylene ammoxidation catalyst. 0 , oxygen responsible for a-H abstraction O, oxygen associated with Mo, responsible for oxygen insertion into the allylic intermediate and , proposed center for O2 reduction and dissociative chemosorption.

It must also be indicated that the presence of water in the reaction gas mixture is essential to obtain high acrylic acid yields over these mixed-metal oxide catalysts. An IR spectra of propylene on these catalysts suggest that jt-allyl intermediate (key for the acrylic acid formation) is not formed in the absence of water, while it is clearly observed when both propylene and water are present ... 

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